This invention relates to purification of phosphoric acid by solvent extraction. More particularly, the invention relates to a method of removing impurities from relatively impure aqueous phosphoric acid solutions by extraction with a substantially water-insoluble solvent.
Aqueous solutions of phosphoric acid have been produced by acidulation of phosphate rock with, for example, sulfuric acid, and recovery of the resultant phosphoric acid together with a variety of impurities. Such solutions, called "wet process acids" have been used in the production of fertilizers.
A number of methods have been suggested for removing at least a portion of the impurities from wet-process acid so as to produce a technical grade acid suitable for a variety of industrial uses, such as pickling of steel, for example. Among these suggested methods, extraction of phosphoric acid using a variety of solvents has been proposed.
U.S. Pat. No. 3,361,527 teaches the use of relatively water-insoluble secondary or tertiary hydrocarbyl amines for extracting phosphate values from wet-process acid. The amines are usually diluted with one or more of a variety of hydrocarbon solvents, or chlorinated hydrocarbon solvents. Optionally, small amounts of aliphatic alcohols can be included to improve the solubility of the amines in the solvents. The use of an amine extractant alone, however, makes recovery of phosphate values in the acid form quite difficult. Usually, an alkaline stripping solution is used, giving a phosphate salt of an alkali metal, sodium phosphate, for example.
U.S. Pat. No. 3,318,661 teaches the extraction of phosphoric acid using as a solvent a dialkyl ether in which each alkyl group contains 2-5 carbon atoms. With an ether solvent alone, however, overall yields of phosphoric acid are low, in the area of 55%, with the remainder reporting to a raffinate stream containing most of the impurities originally present. The ether system also relies on a temperature differential between the extraction step and the acid release step, necessitating refrigeration for optional extraction.
U.S. Pat. No. 3,903,247 shows a mixture of a dialkyl ether with a minor proportion of an aliphatic alcohol of 3-8 carbon atoms. This combination shows an improved split and overall yield of phosphoric acid, but is relatively ineffective in removing anionic impurities such as sulfates from the wet-process acid.
Other references teach the use of still different combinations in purifying phosphoric acid by extraction. U.S. Pat. No. 3,298,782 shows the use of a combination of a major portion of an alcohol with a minor portion of an amine for this purpose. This combination of solvents has not been particularly effective in reducing the level of iron in the product acid, however. The combination of alcohol and amine also appears to be more effective at low (.about.30%P.sub.2 O.sub.5) acid concentrations than at the higher concentrations found in concentrated wet-process acid (.about.60%P.sub.2 O.sub.5).
U.S. Pat. No. 3,972,982 teaches the use of the combination of an amine with a silane compound in extraction-purification of impure phosphoric acid. This latter solvent combination is recommended especially to remove fluorides from the wet-process acid.
In all the above processes, the solvent systems are effective in extracting substantial portions of the phosphoric acid out of the impure wet-process acid. On back-extraction with water, aqueous phosphoric acid of increased purity is obtained. However, in all of these known systems substantial amounts of metal ions are also extracted from the wet-process acid, and show up in the purified acid. A need exists, therefore, for an extraction system which would be more selective to phosphoric acid; that is, one which would remove less of the metallic ion impurities from the wet-process acid and thereby produce a purer aqueous phosphoric acid product.